PROJECT SUMMARY/ABSTRACT Sickle cell disease (SCD) is the most common hemoglobinopathy worldwide. Polymerizing sickle red blood cells undergo intravascular hemolysis and elicit robust inflammation and oxidative stress leading to vessel injury. Indeed, most of the complications of SCD may be related to repeated bouts of tissue ischemia. The physiologic response to vascular injury is the development of collateral vessels, a complex process that requires exquisite interplay between multiple cell types (inflammatory cells, endothelial cells) through tight regulation of chemokines/cytokines and other paracrine cell factors. Reactive oxygen species (ROS) from neutrophils, macrophages and endothelial cells are key mediators of this process. Underproduction of ROS may hamper collateralization, but overabundant ROS has been shown to be detrimental. In addition, the initial inflammatory and oxidative response to ischemia must subside, and a carefully orchestrated program must ensue to promote repair and resolve inflammation to maintain vascular integrity. An essential part of this process is switching of the macrophage phenotype from inflammatory to reparative, which is dependent on efferocytosis of neutrophils. How these processes are regulated in SCD are unknown. Despite the critical importance of collateral formation to preserve end-organ integrity, little is also known about this process in SCD. Using the hind ischemia model, we established that humanized sickle cell mice (SS) exhibit a profound impairment in collateral vessel formation in comparison to their wildtype non-sickle (AA) counterparts. This observation was associated with hyperinflammation, impaired clearance of highly H2O2-producing neutrophils, and inflammatory macrophages that fail to switch to the reparative phenotype. Both the use of a monoclonal antibody to deplete neutrophils and the pharmacologic reduction of ROS with anti-oxidant therapy independently improved collateral vessel formation in the SS mice. Additional work revealed that administration of resolvin D1, a key lipid mediator of the inflammation resolution pathway, also improved collateral vessel formation in the SS mice. These findings suggest that impaired collateral formation in SS mice is likely secondary to dysregulated inflammation-resolution pathways. Aims I and II of this project attempt to identify the mechanisms of hyperinflammation by respectively characterizing the identity and function of neutrophils and macrophages in SS mice during collateral formation. Aim III will utilize lipidomics to determine key defects in the resolvin-synthesis pathway in order to augment inflammation resolution in SS mice. These proposed studies will identify novel targets to promote collateral vessel formation and thereby improve morbidity and mortality in this debilitating disorder with a significant paucity of disease-modifying therapies.